1. Field of the Invention
This invention relates to sheaves and is particularly directed to a multiple sheave assembly.
2. Description of the Prior Art
At present, for such activities of lowering wire cables to which sensors and instruments are attached, as in hydrographic work from ships, a freely suspended, single sheave is employed. In the prior art, line monitors are commonly used to measure cable or line tension, line speed and footage of the cable. An illustrative example of such a system is shown in a brochure entitled "Heck Check Linemonitor", Bulletin HCL 1, manufactured by the Heckerman Corporation of Inglewood, Calif. In these systems a pair of outer pulleys are mounted in offset relation to a center pulley so that the line is bent around the center pulley, the amount of bend being forced to be constant. The amount of downward force applied to the center pulley is a function of the line tension and the bend and is measured by the line monitor. Magnets adjacent to the center pulley are used in the measurement of line footage and speed and a straingauge is used in the measurement of line tension.
There is an inherent disadvantage to these prior art line monitors. The three pulley arrangement severely bends the line in three separate places, leading to wear and damage of the line. This is particularly true with steel cables wherein the individual strands are twisted.
The problems with large single sheave assemblies without counter balancing are described in co-pending application Ser. No. 313,758.
U.S. Pat. Nos. 3,172,642; 3,132,844; 3,042,374; and 3,032,320 disclose various multiple sheave arrangements of the prior art. U.S. Pat. No. 1,348,691 to Beaumont discloses a counter-balanced pulley arrangement that exists in the prior art.
A brochure entitled "The General Oceanics Hydroblock System--Notes on Tension Measurements" by General Oceanics, Inc. of Miami, Fla. illustrates one single pulley system wherein tension measurements were made by using a load cell, i.e., a strain gauge.